CDC researchers say that increase from 1979 to 2003 is probably because more older mothers are giving birth. The prevalence of Down syndrome is five times greater in pregnancies of mothers over age 35.

The number of first births per 1000 women 35 to 39 years of age increased by 36 percent between 1991 and 2001, and the rate among women 40 to 44 years of age rose by 70 percent, according to a 2004 report in the New England Journal of Medicine.

Gov. Sarah Palin had her son Trig last year at the age of 44, an age when the chances of having a child with Down Syndrome is 1 in 35, according to the Eunice Kennedy Shriver National Institute of Child Health and Human Development.

Birthing trends worldwide show that women are waiting longer to have children and advanced maternal age is associated with increased risk of having a child with Down syndrome.

In the December issue of Pediatrics, Mikyong Shin and his colleagues at the CDC reported that the prevalence of Down Syndrome has jumped from 9.0 infants per 10,000 births to 11.8 across 10 geographically, ethnically, and culturally diverse states.

The study is the first broad, population-based effort to look at the prevalence of the syndrome. It conflicts with earlier surveys that projected a decrease in Down syndrome births due to better diagnostic tools and higher abortion rates.

CDC scientists said those numbers are likely to rise over the next several years.

But Dr. Brian Skotko, a pediatric geneticist at Children's Hospital Boston, said the CDC data conflicts with previous data that has demonstrated a decline in the number of Down syndrome births.

Dr. James F.X. Egan and colleagues in the division of maternal-fetal medicine at University of Connecticut Health Center, came to a different conclusions, drawing on data from the National Center of Health Statistics.

They determined that there was a 15 percent decrease in the numbers of babies born with Down syndrome between 1989 and 2005 and that without new testing, there would have been a 34 percent increase in the number of babies born -- an effective 49 percent difference between expected and observed rates.

"The difference likely comes from the data sources that each set of researchers were using," said Skotko. "In either case, the babies born with Down syndrome is far less (49 percent by Egan calculations and 19 percent by CDC calculations) between what could have been and what is actually occurring."

About 400,000 Americans have Down syndrome, the most common genetic condition in the United States, which produces an array of challenges, including retardation, delayed language and slow motor development.

Down syndrome results when an embryo carries three copies of chromosome 21, instead of the normal two copies. The risk of this defect rises as a mother ages.

In 2002, 263 births were reported in women between 50 and 54 years of age.

An estimated 92 percent of all women who receive a prenatal diagnosis
of Down syndrome choose to terminate their pregnancies, according to Skotko's review of research worldwide.

He worries that if Down syndrome cases decline enough in the U.S. funding to study the congenital condition will dry up. There's also worry that more people will deny themselves what some call the "gift" of raising children with Down syndrome.

Nina Fuller of Newburgh, Ind., knew nothing about parenting a child with Down syndrome when she received her diagnosis.

"I presumed she would not be able to communicate, to read, to interact with the rest of our family or with the world around her," she told ABCNews.com. "I was afraid that she would intrude on the lives of her three brothers, and that our family would be home-bound and our lives as we had planned would be thrown into turmoil."

Those fears never materialized. The Fullers went on to adopt another daughter with Down syndrome, Hope.

Parents Need Support, Feedback

Without knowing what it's like to raise a child with Down syndrome, many women will make their decisions based on misinformation -- and myths -- about the disorder, say advocates.

The genetic diagnosis often comes as a shock, and many people assume that raising a child with Down syndrome will be fraught with heartbreak.

Melanie McLaughlin, a Boston mother who decided to raise her child after a Down syndrome diagnosis, said she worried about how the child would affect her siblings and the marriage, and who would look after her when she and her husband died.

But according to a study by Skotko, whose sister has Down syndrome, most siblings are patient and compassionate. As for the marriage, some couples do experience stress in raising a disabled child, but many grow closer.

"I am concerned about mothers making that informed decision," he said. "Are they making it on facts and up-to-date information? Research suggests not, and that mothers get inaccurate, incomplete and sometimes offensive information."

Like Skotko, CDC experts worry about ensuring care for the complex health problems associated with Down syndrome, such as congenital heart defects, as children progress through adulthood.

The CDC study drew on birth defect registries in Arkansas, Georgia, California, Colorado, Iowa, North Carolina, New York, Oklahoma, Texas, and Utah. The regions covered 29 percent of all births in 2002.

Of the estimated 5.5 million live births, 6,580 were diagnosed with Down syndrome. The highest rates were in Utah and the lowest were in Arkansas.

According to the report, crude rates of Down syndrome at birth were slightly higher among boys and Hispanics.